1. Field of the Invention
This invention relates to a sleeve for abutting, aligning, and retaining opposed ferrules for use in an optical connector to be used for connecting optical fibers and a method for the production thereof.
2. Description of the Prior Art
Generally, the connecting part in an optical connector is composed of ferrules having connected thereto a sheathed optical fiber completed by coating the basic thread of an optical fiber with a sheath and a sleeve shaped like a hollow cylinder and adapted to admit opposed ferrules in an aligned state. Particularly unlike the electric connector, the optical connector is required to ensure exact accord between the relative positions of two optical fibers to be connected. It, therefore, becomes necessary to fix an optical fiber in coincidence with the center of a ferrule having the outside diameter thereof and the inside diameter of the part thereof for allowing insertion of the basic thread of an optical fiber finished in respectively specified sizes and then insert a pair of such ferrules into a sleeve through the opposite ends thereof until mutual abutment, and center the axes of the optical fibers. As means for effecting this centering, the methods of the so-called adjusting type which rely on adjusting mechanisms to carry out fine adjustment and the methods of the no-adjusting type which are aimed at heightening the dimensional accuracy of ferrules and sleeves are available. Recently, the methods of the no-adjusting type have been predominating.
Heretofore, most of the ferrules which have been in popular use are those made of such ceramic substances as zirconia. By the same token, the sleeves which are made of such ceramic substances as zirconia have been in popular use.
Published Japanese Patent Application, KOKAI (Early Publication) No. (hereinafter referred to briefly as "JP-A-") 6-27,348, for example, discloses a ceramic sleeve which is formed by providing a tubular body with ridges raised from at least three points on the inner wall surface of the tubular body and extended from one to the other end of the length of the tubular body. The ridge has an upper face formed in a concave circular arc included in a circle centering about the axis of the tubular body, namely a concave arcuate cross section facing the axis of the tubular body. The ridges and the inner wall surface of the tubular body are interconnected with gentle curves. The patent literature mentioned above further discloses a method for the production of the sleeve. This method comprises a step of manufacturing such a ceramic raw material as zirconia or alumina into a tubular body of such a geometric shape as described above, a step of calcining the resultant tubular body, and a step of polishing the upper faces of the ridges on the inner wall surface of the calcined tubular body. When the sleeve is a split type, the method further comprises a step of inserting a slit in the tubular body fresh from the polishing step throughout the entire length thereof in the longitudinal direction.
The ceramic sleeve constructed as described above is generally produced by subjecting the raw material first to primary forming in a cylindrical shape as by powder extrusion or injection molding and then to degreasing and sintering treatments and machining works for grinding the outer surface of the tubular body and abrading the inner wall surface of the tubular body. The process of production, therefore, includes many steps and incurs an enormous cost inevitably. Further, since the raw material is brittle and rigid, the product brings about such problems as shedding chips and leaving the finish of surface polishing at the mercy of the grain size of crystals. Since the ceramic sleeve is rigid and deficient in elasticity, the ridges raised from the inner wall surface of the sleeve tend to inflict scratches on the outer faces of the ferrules and the sleeve and the ferrules, on repeating their mutual attachment and detachment, tend to backlash possibly to the extent of inducing a deviation from the axial alignment of the optical fibers. The ceramic substance, therefore, is not perfectly fit as a material for the sleeve in the optical connector which is prone to frequent attachment and detachment of the ferrules.
Further, since the ceramic sleeve inevitably contracts when it is sintered subsequently to the primary formation, it must be ground to prescribe dimensions by all means. When the ridges are formed as extended in the longitudinal direction on the inner wall surface of the tubular body, therefore, the upper faces of the ridges are ground in a concave arcuate shape along the axis of the tubular body as disclosed in JP-A-6-27,348 mentioned above. When these ridges are formed at three points on the inner wall surface of the tubular body, it is not the concave arcuate faces of the ridges but the opposite lateral edges of these faces in the longitudinal direction that are actually exposed to contact with the outer peripheral surfaces of the ferrules which have been inserted into the sleeve. When the component ridges of the sleeve are exactly in agreement in size, therefore, the sleeve is fated to fix the ferrules in position in a state such that the opposite lateral edges (located at a total of six points) are held in contact with the outer surfaces of the ferrules. When the ridges involve a dimensional error, even if slightly, the contact occurs only at part of the points mentioned above. As a consequence, the possibility arises that the ridges will give rise to a deviation in contact and fixation at the points mentioned above in relation to the ferrules inserted into the sleeve opposite each other and the terminals of the optical fibers being connected consequently will inevitably deviate from their mutual axial alignment.